JPH0634501B2 - Autofocus video camera - Google Patents

Description

The present invention relates to an autofocus system for a video camera that automatically adjusts the focus based on a video signal obtained from an image sensor.

(B) Conventional technology The method of using the video signal itself from the image sensor in the autofocus device of the video camera to evaluate the focus control state has essentially no parallax and has a shallow depth of field. There are many advantages such as being able to focus accurately on a distant subject. Moreover, a special sensor for autofocus is unnecessary, and the mechanism is extremely simple.

Conventionally, an example of this autofocus method is “NHK
A so-called hill-climbing servo control, which is described in "Technical Report" S40, Vol. 17, Vol. 1, Vol. 86, page 21 as "Automatic focus adjustment of TV camera by hill-climbing servo system" by Ishida et al., Is known. . This hill-climbing servo control rotates the lens focus ring so that the amount of high-frequency components of the video signal is always maximized, so the lens does not remain stopped in the out-of-focus state, and it follows very well. It is a good method.

However, this method has a major drawback in that the lens is constantly moved.

One of the drawbacks is that the lens does not stop even when the subject is in focus, so that the shooting screen always shakes even after focusing on a stationary object. The lens currently used in TV cameras changes the focal length by rotating the focus ring,
Therefore, the angle of view of the shooting screen changes. For this reason, in this method in which the focus ring continues to vibrate even after focusing, the screen image becomes very difficult to see because it increases or decreases in a certain cycle.

The second drawback is power consumption. Currently, home video cameras often use a battery as a power source because of their portability, and the focus motor must always be driven. When the above is repeated, more power is consumed than when the motor is rotated in a certain direction due to the inrush current, and the image capturing time becomes shorter. In addition, since the focus ring is always rotated, problems such as wear of gears occur.

The applicant has proposed in Japanese Patent Application No. 61-273212 (H04N 5/232) a new autofocus circuit system for eliminating these drawbacks. The outline of the contents will be described below as a conventional example. In the above application, an extremely fine control method is used to improve the focusing accuracy, but here, only the portions related to the present invention will be described.

FIG. 2 is a block diagram of the autofocus circuit according to the above application.

The image formed by the lens (1) becomes a video signal by the image pickup circuit (4) including an image pickup element and is input to the focus evaluation value generation circuit (5). The focus evaluation value generating circuit (5) is constructed as shown in FIG. 3, for example. The vertical sync signal (VD) and the horizontal sync signal (HD) separated from the video signal by the sync separation circuit (5a) are input to the gate control circuit (5b) to set the sampling area. The gate control circuit (5b) sets a rectangular sampling area in the center of the screen based on the vertical sync signal (VD), horizontal sync signal (HD), and fixed oscillator output, and the luminance signal only within this sampling area. Gate circuit with a gate opening / closing signal that allows the passage of
Supply to (5c).

Only the luminance signal corresponding to the range of the sampling area is high pass filter (HPF) by the gate circuit (5c).
Only the high frequency component is separated after passing through (5d), and amplitude detection is performed by the detection circuit (5e) at the next stage. This detection output is integrated circuit (5f)
After being integrated for each field at, it is converted to a digital value by the A / D converter (5g) to obtain the focus evaluation value of the current field. The focus evaluation value generation circuit (5) configured as described above always outputs the focus evaluation value for one field.

Immediately after the start of the autofocus operation, the first focus evaluation value is held in the maximum value memory (6) and the initial value memory (7). After that, the focus motor control circuit (focus motor control means) (10) rotates the focus motor (3) in a predetermined direction and monitors the output of the second comparator (9). The second comparator (9) compares the focus evaluation value after driving the focus motor with the initial evaluation value stored in the initial value memory (7),
The size is output.

The focus motor control circuit (10) rotates the focus motor (3) in the first direction until the second comparator (9) outputs a large or small output, and the current focus evaluation value is higher than the initial evaluation value. If the output is larger than the preset fluctuation range, the rotation direction is kept as it is and the current evaluation value is smaller than the fluctuation range as compared with the initial evaluation value. When the output is made, the rotation direction of the focus motor (3) is reversed and the output of the first comparator (8) is compared.

The first comparator (8) compares the current maximum focus evaluation value stored in the maximum value memory (6) with the current evaluation value, and the current focus evaluation value is stored in the maximum value memory (6). Of the two comparison signals (S1) and (S2) that are larger than the first threshold value (1st mode) and that are smaller than the preset first threshold value (second mode). Here, the maximum value memory (6) is updated based on the output of the first comparator (8) when the current evaluation value is larger than the content of the maximum value memory (6), and the value is always updated. The maximum evaluation value up to is held.

(13) is a motor position memory that stores the motor position by receiving the motor position signal that indicates the position of the focus motor (3), and the output of the first comparator (8) as with the maximum value memory (6). On the basis of the above, the motor position when the focus evaluation value becomes the maximum value is constantly updated.

The focus motor control circuit (10) monitors the output of the first comparator (8) while rotating the focus motor (3) in the direction determined based on the output of the second comparator (9), In order to prevent malfunction due to noise, a second mode in which the current evaluation value is smaller than the preset first threshold value as compared with the maximum evaluation value is instructed by the output of the first comparator (8). At the same time, the focus motor (3) is reversed. After this reverse rotation, the contents of the motor position memory (13) and the current motor position are compared by the third comparator (14), and when they match,
The focus motor control circuit (10) functions to stop the focus motor (3) when the focus ring (2) returns to the maximum point of the focus evaluation value. At the same time, the focus motor control circuit (10) outputs a lens stop signal (LS).

(11) is the fourth memory that holds the focus evaluation value at that point when the lens stop signal (LS) is issued after the autofocus operation by the focus motor control circuit (10) is finished. This fourth memory (1
The content held in 1) is compared with the current focus evaluation value, and if the value becomes larger than the second threshold value for restart,
Focus motor control circuit assuming that the subject has changed (10)
A subject change signal is output to. Upon receiving this signal, the focus motor control circuit (10) restarts the autofocus operation again and follows the change of the subject.

However, the focus ring (2) does not accurately return to the maximum point of the focus evaluation value for the following two reasons.

One is that it takes a certain time until the focus evaluation value for a certain lens position is output. For example, as for the focus evaluation value at a certain lens position A, an optical image is picked up by the image pickup element at the lens position A for one field period, and the focus evaluation value is output only after reading out as a video signal in the next one field. That is, the focus evaluation value at a certain lens position A is output with a delay of two fields. That is, when the focus motor (3) is moving, the focus ring (2) has already passed the maximum point when the maximum focus evaluation value is output, and when the maximum focus evaluation value is output. If the focus motor (3) is returned to the motor position of, the focus ring (2) will not be fully returned.

The second is from the focus motor (3) to the focus ring (2)
This is due to backlash of the drive system up to, and even if the focus motor (3) is rotated in the reverse direction, the motion is not transmitted to the focus ring (2) for a certain amount after the reverse rotation. Therefore, even if the motor position is returned to the motor position where the focus evaluation value maximum value is recorded without the first factor, the focus ring (2) cannot return to the position where the focus evaluation value maximum value is reached. turn into.

(C) Problems to be solved by the invention In the conventional auto-focus control circuit, when the point where the maximum value of the focus evaluation value is exceeded is exceeded and then it is reversed, the output of the focus evaluation value is delayed and the focus ring drive system becomes loose. for,
In some cases, the focus ring could not be returned to the maximum focus evaluation value, and accurate autofocus operation could not be performed.

(D) Means for solving the problem The present invention is a means for holding the motor position recording the maximum focus evaluation value up to that time, and the value of the motor position memory and the current motor position when the focus ring is returned. The automatic focus operation is performed by a means for detecting the coincidence and a means for stopping the motor after the value of the motor position memory coincides with the current motor position and after a certain value has passed.

(E) Operation Since the present invention is configured as described above, when returning the focus ring to the maximum point of the focus evaluation value, it is possible to return the focus ring to the focal point accurately without causing insufficient return.

(F) Embodiment One embodiment of the present invention will be described below with reference to the drawings. The same parts as those of the conventional example (FIGS. 2 and 3) are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 is a block diagram of an autofocus circuit which is an embodiment of the present invention. The image formed by the lens (1) becomes a luminance signal by the image pickup circuit (4) having an image pickup element, and a focus evaluation value generation circuit (focus evaluation value detection means)
It is input in (5). The focus evaluation value generating circuit (5) is the third one described above.
It has the same configuration as the figure, and the focus evaluation value for one field is output.

The focus motor control circuit (10) monitors the output of the first comparator (8) while rotating the focus motor (3) in the direction determined based on the output of the second comparator (9), and evaluates the value. First comparator (8) to prevent malfunction due to noise
At the same time, the focus motor (3) is rotated in the reverse direction at the same time when the output indicates the second mode in which the current evaluation value is smaller than the preset first threshold value as compared with the maximum evaluation value. After this reverse rotation, the contents of the motor position memory (13) and the current motor position are compared by the third comparator (motor position comparing means) (30). At the same time, the motor return correction amount holding circuit (15) is input to the third comparator (30), and the third comparator (30) compares the contents of the motor position memory (13) with the current motor position and both The motor position coincidence signal is not moved until the motor position coincidence signal is reached by the focus motor control circuit (10) until the motor position moves excessively by the motor return correction amount holding circuit (15)
Output to. Here, the focus motor control circuit (10) functions to stop the focus motor (3) when the focus ring (2) returns to the maximum point of the focus evaluation value, and at the same time outputs the lens stop signal (LS).

Here, the motor return correction amount (15) is the sum of the amount of movement of the focus motor (3) during the delay of the focus evaluation value output and the amount of backlash between the focus motor (3) and the focus ring (2). It is set. In order to operate as described above, the focus ring (2) accurately returns to the position showing the maximum value of the focus evaluation value, and an accurate autofocus operation can be realized.

(G) Effect of the Invention As described above, according to the present invention, when the focus ring goes past the point showing the maximum of the focus evaluation value and then returns, the output of the focus evaluation value is delayed or the focus motor and the focus ring are separated from each other. Accurate autofocus operation can be realized without lack of return due to play.

[Brief description of drawings]

FIG. 1 is a circuit block diagram of an embodiment of the present invention, FIG. 2 is a circuit block diagram of a conventional example, and FIG. 3 is a circuit block diagram of an evaluation value detection circuit. (1) …… Lens, (3) …… Focus motor, (5) …… Focus evaluation value generation circuit (evaluation value detection means), (10) …… Focus motor control circuit, (13) …… Motor position memory , (30)
...... Third comparator (motor position comparison means), (15) ... Motor return correction amount holding circuit.

Claims (1)

[Claims] 1. A focus evaluation value detecting means for outputting a high frequency component level of a luminance signal obtained from an image pickup device as a focus evaluation value, a focus motor for displacing a focus lens, and the focus evaluation value being a maximum evaluation value. A motor position memory that stores the motor position of the focus motor at that time; and a focus motor control circuit that reverses the focus motor when the focus evaluation value reaches a maximum evaluation value and then decreases by a preset threshold value or more, After the current motor position coincides with the value of the motor position memory when the focus motor is rotated in the reverse direction, it is further changed by a predetermined amount corresponding to a response time delay from the focus evaluation value detection means to the drive mechanism of the focus lens. A motor position comparison means for detecting is provided, and the motor position is detected by the motor position comparison means. Autofocus video camera, wherein the focus motor that allowed to stop when it is detected that changes by quantitative.